In general, a compressor is a mechanical apparatus that receives power from a power generation apparatus such as an electric motor, a turbine or the like and compresses air, refrigerant or various operation gases to raise a pressure. The compressor has been widely used in an electric home appliance such as a refrigerator and an air conditioner, or in the whole industry.
The compressor is roughly classified into a reciprocating compressor wherein a compression space to/from which an operation gas is sucked and discharged is defined between a piston and a cylinder, and the piston is linearly reciprocated inside the cylinder to compress refrigerant, a rotary compressor wherein a compression space to/from which an operation gas is sucked and discharged is defined between an eccentrically-rotated roller and a cylinder, and the roller is eccentrically rotated along an inner wall of the cylinder to compress refrigerant, and a scroll compressor wherein a compression space to/from which an operation gas is sucked and discharged is defined between an orbiting scroll and a fixed scroll, and the orbiting scroll is rotated along the fixed scroll to compress refrigerant.
Particularly, the rotary compressor has been developed to a twin rotary compressor, wherein two rollers and two cylinders are provided at upper and lower portions, and the pairs of rollers and cylinders of the upper and lower portions compress some and the other of the entire compression capacity, and a 2 stage rotary compressor, wherein two rollers and two cylinders are provided at upper and lower portions, and the two cylinders communicate with each other so that one pair can compress relatively low pressure refrigerant and the other pair can compress relatively high pressure refrigerant passing through a low pressure compression step.
Korean Registered Patent Publication 1994-0001355 discloses a rotary compressor. An electric motor is positioned in a shell, and a rotation axis is installed to pass through the electric motor. In addition, a cylinder is positioned below the electric motor, and an eccentric portion fitted around the rotation axis and a roller fitted onto the eccentric portion are positioned in the cylinder. A refrigerant discharge hole and a refrigerant inflow hole are formed in the cylinder, and a vane for preventing non-compressed low pressure refrigerant from being mixed with compressed high pressure refrigerant is installed between the refrigerant discharge hole and the refrigerant inflow hole. Moreover, a spring is installed at one end of the vane so that the eccentrically-rotated roller and the vane can be continuously in contact with each other. When the rotation axis is rotated by the electric motor, the eccentric portion and the roller are rotated along the inner circumference of the cylinder to compress refrigerant gas, and the compressed refrigerant gas is discharged through the refrigerant discharge hole.
Korean Laid-Open Patent Publication 10-2005-0062995 suggests a twin rotary compressor. Referring to FIG. 1, two cylinders 1035 and 1045 for compressing the same capacity and a middle plate 1030 are provided to improve a compression capacity twice as much as that of an 1 stage compressor.
Korean Laid-Open Patent Publication 10-2007-0009958 teaches a 2 stage rotary compressor. As illustrated in FIG. 2, a compressor 2001 includes an electric motor 2014 having a stator 2007 and a rotor 2008 at an inside upper portion of a hermetic container 2013, and a rotation axis 2002 connected to the electric motor 2014 includes two eccentric portions. A main bearing 2009, a high pressure compression element 2020b, a middle plate 2015, a low pressure compression element 2020a and a sub bearing 2019 are successively stacked from the side of the electric motor 2014 with respect to the rotation axis 2002. In addition, a middle tube 2040 is installed to introduce refrigerant compressed in the low pressure compression element 2020a into the high pressure compression element 2020b. 
In the conventional twin rotary compressor, an area of a discharge port formed in a muffler is equal to the sum of areas of respective discharge ports discharging refrigerant compressed in the two cylinders. Moreover, in the conventional 2 stage rotary compressor, an area of a discharge port formed in a muffler is equal to or larger than the sum of areas of a first discharge port and a second discharge port, or equal to or larger than the double of the area of the first discharge port as in the twin rotary compressor. Accordingly, a volume flow of refrigerant discharged in a discharge stroke of a 2 stage compression assembly of the 2 stage rotary compressor, and the area of the discharge port of the muffler are not optimized to thereby entirely increase a noise spectrum.